michaelis



June 20, 1961 J, MICHAELIS Re. 25,000

NOVEL APPARATUS Original Filed Dec. 5, 1956 IN V EN TOR. JOHN L. MIC/IA!IS nrram ly plus or minus about 0.05 volt.

United States Patent This invention relates to apparatus for supplyingdirect current at amperages in excess of 10,000 amperes, frequentlyabove 20,000 amperes, at voltages in the range of 100 to 500 volts orabove from an alternating current power source. It is especially adaptedfor supply of cur rent to a series of electrolytic cells such as aseries of alkali metal chloride cells which are used for the productionof chlorine and caustic soda.

' 'In'thc' performance of various chemical reactions such as in theproduction of chlorine and caustic soda by electrolysis of sodiumchloride a series of electrolytic cells are connected together andenergized by a direct current .power source. These cells operate at arelatively low voltage (usually below) about volts and in recent yearshave been developed to operate at a very high amperage normally 10,000to 20,000 amperes or more. Heretofore the current has been supplied tosuch cells by rectification of alternating current by means ofmechanical or mercury arc rectifiers. The high internal resistance ofsuch rectifiers can result in substantial loss of power particularlywhen used in connection with cells having the high current demandreferred to above.

The present invention permits use of semiconducting rectifiers whichoperate at power efficiencies in excess of 95 percent and which haverelatively low internal resistance, generally below about 0.02 ohm. Suchrectifiers have internal voltage drops below one volt frequently in therange of 0.42 to 0.54 volt. By use of such semiconducting rectifiers asherein contemplated, a substantial saving in power is effected.Rectifiers of the type herein contemplated are those which rely upon thesemiconductor properties of certain metalloids such as metallic gerorsilicon.

The unusually low resistance and high efficiency of the germanium andsilicon rectifiers makes their use attractive. However, individualrectifiers can supply only a small amount of current in the range ofabout 50-500 amperes. Hence, I have found that it is necessary to use a.large number of such rectifiers coupled in parallel to meet the highcurrent demand.

In such a case, however, the unusually low internal resistance ofgermanium and like rectifiers creates a problem. Inevitably, the variousrectifiers which are coupled together vary in the magnitude of theirinternal resistance.

The variation in terms of internal voltage drop from rectifier torectifier is below plus or minus 0.25 volt, usual- Consequently, whenconnected in parallel, those rectifiers in the parallel circuit havingmaterially lower resistances than the others pass a disproportionateshare of the load current and become inefficient or inoperative due tooverload and subsequent failure.

The present invention affords a simple and convenient means for avoidingthese difficulties. According to this invention an alternating currentpower supply is coupled in [parallel] series with a plurality ofcircuits each containing a semiconducting rectifier having an internalresistnnce below 0.02 ohm such as a germanium or silicon rectifier. Alsoin series with the rectifier in each circuit on the alternating currentside thereof is a small electromo- Re. 25,00i Reissued June 20, 1961tive force. The magnitude of this electromotive fore normally is quitesmall frequently being of the order 0 0.2 volt or less per rectifierelement or diode and is sucl that it adds to or subtracts from theimpressed alternatin; current voltage an amount of electromotive forcesufli cient to compensate for the differences in internal resist ancebetween the rectifiers in the several circuits am thereby to balance thecircuits and to avoid overloading of individual rectifiers.

In the practice of this invention, a system is employcc which maycomprise a single or a relatively large numbe of balanced alternatingcurrent outputs, each of these out puts being used to energize a singlerectification circuit The rectification circuit may consist of a singlediode or a bridge circuit employing a plurality of diodes in serie: or aplurality of such bridge circuits in parallel. All 01 theserectification circuits supply the common load. 11 all the alternatingcurrent outputs were maintained a an equal potential, each rectificationcircuit fed therefron would be forced to carry a more equal share oftheloac' current. Such a number of alternating current outputs a:satisfactory voltages are obtained in the present systerr from aplurality of transformers or a plurality of wind ings on the sametransformer. It is not practical, how

ever, with ordinarytransformers alone to achieve any high degree ofidentity to output voltage without going to great expense, inmanufacture. Induced voltages will vary slightly from winding towinding; and, furthermore, at such high currents as will be met in theabove-mentioned rectification systems, the differences in transformerinternal impedance as seen by the load become especially important.Ordinary transformers will match impedance: with 7% percent if thetransformers are manufactured to identical mechanical and electricalspecifications. It one transformer is Wound with a number of secondarywindings, it is still difiicult to match the impedancw ol thesecondaries within 5 percent Without extra manufacturing costs.Furthermore, if it is desired that the secondaries feed rectifiershaving somewhat unforeseen characteristics with identical currents,manufacturing exactness of the antecedent transformers will not achievethe desired result. M

When a plurality of transformers are used to supply the alternatingcurrent power for rectification the variation from transformer totransformer may be dealt with by providing a plurality of taps off theprimary or secondary winding so that the output of the transformer maybe at different voltages depending upon the voltages between taps. Byproviding a number of taps which have relatively small voltagestherebetween, it is possible to couple all of the transformers atidentical voltage output simply by selecting the proper taps from eachtransformer.

In any event whether one or many secondary windings are used, it isnecessary to provide balancing electromotive force in series with eachrectifier bridge in order to compensate for variation in rectifiers. Aneffective balancing apparatus for practice of this invention includes anautotransformer energized by alternanitg current of frequency and phasesubstantially identical with that of the alternating current source.This autotransformer has a number of tapped connections wherewithportions of the autotransformer may be connected serially with therectifier and the windings of the power source. It is preferable andeconomical to employ a three-phase alternating current source and athree-phase autotransformer in this system, although single-phasesystems are also practical. The alternating current source employedwould ordinarily be a transformer, but could conceivably be anarternating current generator or other source. i

The invention will be described with reference toth and in physicalsize, that is, say 3000 amperes at .14 volt is 420 volt amperes but onlya reduced ampere capacity at the end taps is required. Thus over-allthis transformer may be about .2 k.v.a. and relatively low in cost,especially since three .2 k.v.a. transformers are required for the one1,125 k.v.a. three-phase rectifier power transformer. The unbalancedpotential in the ten parallel circuits is so small and it is impracticalto provide 100 power transformers for this type installation.

Balancing is required between the ten 1,125 k.v.a.

power transformers because of their unequal impedance.

Six V4 of 1% taps are suggested on the primary winding of these tenpower transformers to permit adjustment that will provide equal loadcarrying capability for each of these ten power transformers.

While an autotransformer effectively serves to supply the balancingpotential herein sought, other means of supplying such potential may beresorted to within the purview of this invention. Thus, in lieu of anautotransformer, a transformer having primary and secondary windings isenergized independently of transformer 130. In such a case, pluralitiesof taps from the secondary winding are provided to supply the smallbalancing electromotive force required.

Although the present invention has been described with reference to thespecific details of certain embodiments, it is not intended that suchdetails shall be regarded as limitations on the scope of the invention,except insofar as included in the accompanying claims.

What is claimed:

1. A direct current power source capable of supplying a load of at least10,000 amperes which comprises an alternating current power source, aplurality of individual circuits coupled in parallel, each of saidindividual circuits comprising a small alternating electromotive forcein series with a rectifier having an internal resistance less than 0.02ohm, [each of] said individual circuits coupled in [parallel] serieswith said alternating current source to provide a source capable ofsupplying a direct current, the magnitude and polarity of each of saidelectromotive forces being such as to compensate for the differences ininternal resistance in the respective rectifiers of said individualcircuits and to provide substantially the same load current through eachof said rectifiers.

2. A direct current power source capable of supplying a load of at least10,000 amperes which comprises an alternating current power source, aplurality of individual circuits coupled in parallel, each of saidindividual circuits comprising a small alternating electromotive forcein phase with said alternating current power source and in series with arectifier having an internal resistance less than 0.02 ohm, [each of]said individual circuits coupled in [parallel] series with saidalternating current source to provide a source capable of supplying adirect current, the magnitude and polarity of each of said electromotiveforces being such as to compensate for the differences in internalresistance in the respective rectifiers of said individual circuits andto provide sub stantially the same load current through each of saidrectifiers, and means to connect said direct current power source to aplurality of electrolytic cells in series.

3. A direct current power source capable of supplying a load of at least10,000 amperes which comprises an alternating current power source, aplurality of individual circuits coupled in parallel, each of saidindividual circuits comprising a small alternating electromotive forcein series with a germanium rectifier having an internal resistance lessthan 0.02 ohm, [each of] said individual circuits coupled in [parallel]series with said alternating current source to provide a source capableof supplying a direct current, the magnitude and polarity of each ofsaid electromotive forces being such as to compensate for thedifferences in internal resistance in the respective rectifiers of saidindividual circuits and to provide substantially the same load currentthrough each of said rectifiers.

4. A direct current power source capable of supplying a load of at least10,000 amperes which comprises an alternating current power source, aplurality of individual circuits coupled in parallel each of saidindividual circuits comprising a small alternating electromotive forcein series with a silicon rectifier having an internal resistance lessthan 0.02 ohm, [each of] said individual circuits coupled in [parallel]series with said alternating current source to provide a source capableof supplying a direct current, the magnitude and polarity of each ofsaid electromotive forces being such as to compensate for thedifierences in internal resistance in the respective rectifiers of saidindividual circuits and to provide substantially the same load currentthrough eachof said rectifiers.

5. The power source of claim 1 wherein the magnitude of saidelectromotive force is not in excess of 0.3- volt per rectifier unit inseries therewith.

6. The power source of claim 3 wherein the magnitude of saidelectromotive force is not in excess of 0.3 volt per rectifier unit inseries therewith.

7. An electric power source capable of supplying low voltage, highamperage direct current which comprise: an alternating current winding,a plurality of [rectifiers] rectifier circuits in series with saidwinding said circuit: having a plurality of semiconductor rectifiersinserie: therein, and thereby providing a direct current potential saidcircuits being coupled in parallel, and means for balancing the directcurrent potentials produced by sair rectifier circuits [winding]comprising an autotransforme: having a winding in phase with saidalternating curren winding and means for supplying balancing additive amsubtracting potential to said rectifier circuits [alternatim currentwinding] from said autotransformer.

[8. An electric power source capable of supplying low voltage highcurrent energy comprising an alternatin, current transformer having aprimary winding and a sec ondary winding, an autotransformer connectedto an deriving power from said secondary winding, said autc transformerhaving a plurality of taps and rectificatio means for convertingalternating current into direct cur rent connected between various ofsaid taps] [9. The power source of claim 8 wherein said rectif cationmeans comprises a bridge circuit having a died in each leg thereof andhaving output connections fc supplying energy to a load.]

10. A direct current power source capable of supplr ing a load of atleast 10,000 amperes which comprises a alternating current power source,a plurality of rectifier in separate circuits, said rectifiers havingindividual i1 ternal resistances less than 0.02 ohm, an autotransformetaps ofi the autotransformer supplying an electromotir force from theautotransformer, each of said circui being coupled in series with thealternating current pow source and one of said taps, said circuits beingcouple in parallel, the magnitude of the electromotive force su; pliedby said taps being such as to compensate for t] difference in internalresistance between circuits and provide substantially the same loadcurrent through eat of the rectifiers.

11. The power source of claim 10, wherein the mag] tude of theelectromotive force supplied by each of sa taphis not in excess of 0.3volt per rectifier in series ther W1 12. The power source of claim 1wherein the sun electromotive force is supplied from an energy sourindependent of said alternating current power sour 13. The power sourceof claim 1 wherein the sm. alternating electromotive force is in phasewith the altl nating current source.

14. The power source of claim 1 wherein said alti nating current powersource comprises a plurality separate transformers the output of eachtransforrr being cofihct d t o at ieast two-cf said ihdivi dugl i A d et r -p merw r e r mbl fq uep ying a load at least 10,000 'amperes whichcomprises dividual circuits coupled in parallel each of said circuitscomprising a small electrornotive force inseries with a plurality ofrectifiers, each of said' rectifiers having an internal resistance ofless than 0.02 ohm said circuits being coupled in series with saidalternating current source to provide a source capable ofsupplyingadirect current, the magnitude and-polarity of each of said'electrgrnotive forces being such as to compensate for the dfi rww i tnal re e the e e t fect janj alternating current power source, aplurality of in- V fiers ofsaid circuits and to provide substantiallythe'same load current through each of said rectifiers. 7 A

16. An electric power source capable of supplying low voltage, highamperage direct current which comprises an alternating currenttransformer having a primary winding and a secondary "winding, aplurality of rectifier circuits'in serieswith' 'said secondarywindingye'ach of said circuits having a plurality of semiconductingrectifiers in series therein and thereby providing a direct currentpotential, said circuits being coupled in parallel and anautotransformer connected to and deriving power from said secondarywinding, said autotransformer having a plurality 0 f taps connected tosaid rectifier circuits for supplying,balancing additive and subtractingpotential to said rectifier circuits.

17. The power source of claim 16 wherein the rectifier circuits comprisebridge circuits having a diode in each leg thereof and having outputconnections for supplying energy to a load.

References Cited in t he file of this patent or the original patentUNITED STATES PATENTS "2,800,621 Carlson et a1. July 23, 1957 FOREIGNPATENTS 846,724 Germany Aug. 18, 1952

